Spatial light modulators (SLM) have been around for over 25 years. Recent advancements have allowed the SLM to be used in a wide variety of fields, such as microscopy, holographic imaging, data storage, DNA synthesis, offset printing, and image projection, to name a few. Using technology developed for the microchip fabrication industry, the SLM has become one of the most successful micro-electro mechanical systems. One form of SLM, a Digital Micromirror Device (DMD) has over 1 million mirrors arrayed on a small chip.
A DMD can have a two dimensional array of microscopic mirrors. In a typical DMD, each mirror may have an area of 16 square micrometers. The mirrors are spaced only 1 micrometer apart. Each mirror can be attached to a micro-electronic (MEMS) device and a hinge, allowing a computer to control the direction that each mirror is pointing. Due in part to the mirror's extremely small size, the direction it is facing can be changed thousands of times per second. The mirror is considered to be in an “on” position when the mirror reflects light onto a display screen. The mirror is off when the light is not reflected. By controlling the percentage of time that the mirror is on or off, at least 1024 shades of gray can be shown on the screen for each mirror. With an array of 1000×1000 mirrors, the DMD can project a video image with a million pixels. Digital projectors using DMD technology are now used to show movies with unprecedented clarity. DMD chips are also used in high definition large screen televisions and lightweight digital video projectors used in offices and home theaters. However, the chip's binary design, allowing each mirror to be either on or off, creates a limitation in its ability to reproduce color images.
Presently, two methods are used in most systems for colorizing a DMD projected image. The first method involves splitting white light using a prism into its red, blue, and green components. Each color is then input into its own DMD chip, with the three outputs directed so that their combined image appears to be full color to the human eye. This method works well for high quality expensive display devices such as Movie Theater projectors. However, the expense of using three chips, combined with the necessary opto-mechanical structure to focus and align the three outputs, creates a system that is prohibitively expensive to be used in consumer applications.
A second method for colorizing a DMD projected image attempts to overcome the expense of using three chips. In the second method, a single DMD chip is used with a transparent rotating color wheel between the light source and the DMD chip. The color wheel's rotation is synchronized with the movement of the micromirrors, allowing a micromirror to turn on when the correct color is shining through the color wheel. Using a red, green, and blue color wheel, each color is able to shine on the mirror ⅓ of the time. By rotating the color wheel fast enough, a red, green, or blue pixel can be displayed on the projection screen when needed, allowing a full color image to be produced.
There are at least two limitations that narrow the DMD's use in consumer applications, however. First, the use of a sequential color wheel limits the overall brightness of each color to less than ⅓ of the intensity of the light source. The brightness is decreased because each color can only shine through ⅓ of the time in a color wheel with 3 colors. Second, due to the sequential nature of the color wheel system, visual artifacts appear on the display screen caused by the strict periodic ordering of the sequential system. These limitations narrow the usefulness of a DMD chip in a high definition projection system, since brightness and clarity are two of the most important aspects of such a system.